(1) Field of the Invention
The present invention relates to an electro-oculography measuring device which estimates a drift component, which is a baseline drift noise, from an electro-oculography original signal of a user, so as to output an electro-oculography signal from which an influence of a drift is removed.
(2) Description of the Related Art
In recent years, there have been presented gaze-path input interfaces and the like which utilize human eyeball movement. Methods for detecting human eyeball movement include: an EOG method that utilizes a potential generated between a cornea and a retina; a corneal reflex method that detects movement of a virtual image generated inside a cornea by irradiating an eyeball with a spotlight; a limbus tracking method that uses a difference in reflectance between a cornea and a sclera; a method using a contact lens; and so on.
Here, the EOG method is a method for detecting eyeball movement, utilizing the fact that a human cornea is charged positively with respect to the retina. More specifically, electrodes are attached near a human eyeball and a change in the potential measured by the electrodes is used to detect eyeball movement. FIG. 64A and FIG. 64B show examples of the method for detecting eyeball movement using the EOG method. FIG. 64A and FIG. 64B are examples of the case where electrodes are attached on the inner and outer sides of the right eye of a user with an equal distance from the center of the eyeball.
Assuming the electro-oculogram generated in the outer-side electrode A is Va and the electro-oculogram generated in the inner-side electrode B is Vb, Va and Vb are equal when the eyeball of the user is at the center as in FIG. 64A, and an electro-oculogram Va-b of 0 V is thereby measured. On the other hand, in the case where the user looks to the right as in FIG. 64B, the electrode A becomes closer to the cornea of the right eye, and thus Va becomes greater than Vb and the measured electro-oculogram Va-b indicates a plus value. Conversely, in the case where the user looks to the left, Va becomes smaller than Vb and the measured electro-oculogram Va-b indicates a minus value. Thus, it can be observed that the user's eye has moved to the right when the measured electro-oculogram Va-b indicates a plus value, and that the user's eye has moved to the left when the measured electro-oculogram Va-b indicates a minus value. With the EOG method, eyeball movement of the user is detected by utilizing such changes in the measured electro-oculogram Va-b as described above.
When detecting eyeball movement using the EOG method, a phenomenon called a drift occurs. A drift is a phenomenon in which the baseline of an electro-oculography original signal changes temporally, and it is considered to be caused by such factors as the materials of the electrodes used for measuring the electro-oculogram and a change in the state of contact between the skin and the electrodes.
FIG. 65 shows a result of electro-oculography measurement performed using electrodes actually attached to a user. FIG. 65 shows a result of electro-oculography measurement when a plurality of indexes are displayed for one second each in the order shown in FIG. 66. Referring to FIG. 65, it can be seen that the baseline of the measured electro-oculogram changes with time. More specifically, due to an influence of the drift, a change occurs in the measured electro-oculogram also when the user gazes at the index (R4). That is to say, the detection of the user's eyeball movement becomes erroneous if the measuring result including a drift is used as the electro-oculogram.
Patent Literatures 1 and 2 below disclose an example of a method of removing the drift component.
With the method of removing the drift component disclosed in Patent Literature 1, the user is instructed to gaze at a particular position (the center of a display, for example) for such a short time that the influence of the drift does not occur, and a value of fluctuations caused by the drift is updated every time the user is instructed to gaze at a particular position, to thereby remove the drift component.
With the method of removing the drift component disclosed in Patent Literature 2, a calibration symbol is generated on a display screen as necessary. When saccadic movement is detected within a predetermined time period, it is determined that the user has gazed at the calibration symbol, and an error caused by the drift component is reset.